The present invention generally relates to a method and an apparatus for controlling the air-fuel ratio of a gas mixture of air and fuel supplied to an internal combustion engine through a feedback control by utilizing the output of an air-fuel ratio sensor. More particularly, the invention is concerned with an air-fuel ratio control method and apparatus which are capable of detecting malfunction or degratation of the air-fuel ratio sensor.
In conjunction with the control of the fuel supplied to cylinders of an internal combustion engine, there has been heretofore known and adopted extensively in practical applications a method of controlling the air-fuel ratio of the mixture gas supplied to the engine through feedback control as well as an apparatus for carrying out the method based on the detection of the intake air amount and the air-fuel ratio detected by an air-fuel ratio sensor (such as O.sub.2 -sensor or the like) installed in an exhaust pipe of the internal combustion engine.
By the way, increasing interest concerning environmental problems and reinforced regulations concerning the exhaust emission of motor vehicles as well as activities directed to improvement of fuel-cost performance of the motor vehicle in recent years require more accurate control of the air-fuel ratio and appropriate detection of unsatisfactory operations or malfunctions of various detecting devices employed in the control of the air-fuel ratio.
As an approach for satisfying the demand mentioned above, there has been proposed a method of determining the performance of an exhaust gas concentration detector, such as an O.sub.2 -sensor for the purpose of judging or detecting the unsatisfactory operation or malfunction of the O.sub.2 -sensor which is employed for detecting the actual air-fuel ratio by sensing the concentration of oxygen contained in the exhaust gas of the internal combustion engine, as is disclosed in JP-A-62-119450. According to this prior art sensor performance or function diagnosis method, the air-fuel ratio of the fuel gas mixture supplied to the engine is varied in the manner of a rectangular waveform, wherein the response rate of the O.sub.2 -sensor detected upon change of the air-fuel ratio is determined. More specifically, a time lag T.sub.RL in the output of the O.sub.2 -sensor produced the fuel concentration of the mixture gas supplied to the engine changes from a low to a high level is compared with a time lag T.sub.LR accompanying the output of the O.sub.2 -sensor produced upon change of the fuel concentration from a high to a low level, whereon a decision is made on the basis of the result of comparison as to whether or not the O.sub.2 -sensor is operating satisfactorily.
The prior art O.sub.2 -sensor performance diagnosis or decision method described above is however disadvantageous in that the torque generated by the internal combustion engine is subjected to significant variation because of a remarkable change (from 13.1 to 16.1) of the air-fuel ratio of the mixture gas supplied to the engine upon making a decision concerning the performance of the air-fuel ratio sensor. As a consequence, when the decision of sensor performance is carried out in the course ordinary running of a motor vehicle equipped with the internal combustion engine of concern, the driver feels a shock to his or her senses, which in turn means that the maneuverability of the motor vehicle is degraded. For this reason, a restriction or limitation is necessarily imposed on the timing for making a decision as to the performance of the air-fuel ratio sensor, giving rise to a problem.